Various kinds of processes such as film formation, etching, oxidation and diffusion are performed on a processing target object such as a semiconductor wafer or a liquid crystal substrate. These processes are performed in a processing chamber. There exist various types of transfer devices configured to load the processing target object into the processing chamber or unload it from the processing chamber. Especially, a multi-joint robot that extends and contracts like frog's legs is widely used. Since such a frog-leg type multi-joint robot does not use a timing belt, it has high resistance against high vacuum and heat.
Patent Document 1 describes a frog-leg type multi-joint robot. The frog-leg type multi-joint robot has four links (two driving arms and two links) assembled in a rhombus shape. The two driving arms are connected to a central hub to be rotated about the central hub. One end of each of the two links is rotatably connected to a leading end of each of the two driving arms, respectively. Further, the other end of each of the two links is rotatably coupled to a plate-shaped holding table for holding thereon a processing target object. In this frog-leg type multi-joint robot, by rotating the two driving arms in opposite directions, the holding table is moved away from or close to the central hub in a radial direction.
In the frog-leg type multi-joint robot, only with the configuration where the holding table is rotatably connected to the leading ends of the two links, the holding table cannot have a stable posture. In order for the holding table to have the stable posture, a gear is fastened to a connecting shaft of the leading end of each of the two links. The two gears fastened to the two connecting shafts are engaged with each other, and the two links are allowed to be moved synchronously. With this configuration, the holding table fastened to the leading ends of the two links can have a stable posture.
Patent Document 2 describes another example frog-leg type multi-joint robot. In the multi-joint robot described in Patent Document 2, in order to control the posture of the holding table, steel belts instead of gears are wounded around two connecting shafts in a cross shape. By the steel belts, the two connecting shafts are synchronized to rotate in opposite directions. Accordingly, the holding table fastened to the leading ends of two links can have a stable posture.
Another example frog-leg type multi-joint robot is described in Patent Document 3. In this frog-leg type multi-joint robot, a direction maintaining unit is added to four links assembled in a rhombus shape in order to control the posture of the holding table.
The direction maintaining unit includes a direction setting arm shaft fastened to the holding table. The direction setting arm shaft is elongated toward a central hub from the holding table. The direction setting arm shaft is provided between a pair of guide magnets and held by a repulsive force therebetween. When the holding table is moved away from the central hub or close to the central hub in a radial direction, the direction setting arm shaft is moved in its axial direction with respect to the pair of guide magnets while still maintained between the pair of guide magnets. Accordingly, the holding table fastened to the direction setting arm shaft can have a stable posture. Further, in Patent Document 3, it is also mentioned that a shaft and a ball spline configured to be rotatable or slidable on the shaft may be used instead of the direction setting arm shaft and the guide magnets.
Patent Document 1: Japanese Patent Laid-open Publication No. H06-015592
Patent Document 2: Japanese Patent Laid-open Publication No. H07-504128
Patent Document 3: Japanese Patent Laid-open Publication No. H11-313477 (see paragraphs [0108] to [0115] and FIG. 7)